Field of the Invention
The invention pertains to memory cells and to the production of such memory cells.
In order to store large volumes of data, for example in data processing (DP) applications or for the digital storage of music or images, use is mainly made at present of memory systems which have mechanically moving parts such as, for example, hard disk storage, floppy disks or compact discs. The moved parts are subject to mechanical wear. Furthermore, they require a comparatively large volume and permit only slow data access. Moreover, since they are sensitive with regard to vibrations and positioning, and they have a comparatively high power consumption for their operation, these memory systems can be used in mobile systems only to a limited extent.
Relatively small volumes of data are usually in semiconductor-based read-only memories (ROM). These are often realized as planar, integrated silicon circuits in which MOS transistors are used as memory cells. The transistors are selected via the gate electrode which is connected to the word line. The input of the MOS transistor is connected to a reference line and the output is connected to a bit line. An assessment is made during the read operation, as to whether or not a current is flowing through the transistor. Logic values zero and one are assigned correspondingly. The storage of zero and one is effected in technical terms in that no MOS transistor is produced or no conductive connection to the bit line is realized in memory cells in which the logic value assigned to the state "no current flow through the transistor" is stored. As an alternative, MOS transistors which have different threshold voltages due to different implantation in the channel region can be realized for the two logic values.
The semiconductor-based memories permit random access to the stored information. The electrical power required to read the information is distinctly less than in the case of the above-mentioned memory systems which have mechanically moved parts. Since no moved parts are required, concerns with regard to mechanical wear and sensitivity to vibrations do not apply here. Semiconductor-based memories can therefore be used for mobile systems as well.
The above-mentioned silicon memories have a planar structure. A minimum area requirement thus becomes necessary for each memory cell and is 4 F.sup.2 in the most favorable case, where F is the smallest structure size that can be produced with the respective technology.
The use of vertical MOS transistors in a read-only memory is disclosed in U.S. Pat. No. 4,954,854 to Dhong et al. The surface of a silicon substrate is provided with trenches, in that system, which are adjoined at the bottom by a source region, which are adjoined at the surface of the substrate by a drain region, and along whose sides a channel region is arranged. The surface of the trench is provided with a gate dielectric and the trench is filled with a gate electrode. Zero and one are differentiated in that configuration in that no trench is etched and no transistor is produced for one of the logic values.
A read-only memory cell configuration whose memory cells comprise MOS transistors is disclosed in German patent publication DE 42 14 923 A1. There, MOS transistors are arranged along trenches in such a way that a source region adjoins the bottom of the trench, a drain region adjoins the surface of the substrate, and a channel region adjoins at the side and bottom of the trench both vertically with respect to the surface of the substrate and parallel to the surface of the substrate. The surface of the channel region is provided with a gate dielectric. The gate electrode is designed as a spacer (edge cover). The logic values zero and one are differentiated by different threshold voltages, which are effected by channel implantation. During the channel implantation, the implanting ions impinge on the surface of the respective trench at such angles that implantation is deliberately effected only along one side due to shading effects of the opposite side.